The present invention is concerned with an anti-lock brake system for use with automotive vehicles and, more particularly, to such a system comprising a master brake cylinder coupled upstream of which is a vacuum brake force booster and means for generating a restoring force counter-acting the brake applying force. The means for generating a restoring force is caused by vacuum and is controllable with the aid of multi-way valves.
A brake system of the afore-described type is disclosed, for example, by U.S. Pat. No. 4,702,531 issued Oct. 27, 1987 (corresponding to German DE-Offenlegungsschrift No. 33 17 629) and U.S. Pat. No. 4,659,153 issued Apr. 21, 1987 (corresponding to German DE-Offenlegungsschrift No. 34 28 869). These systems in the event of an excessive pedal force during an anti-lock controll will generate a counter-acting force by changing or even inverting the pressure conditions in the vacuum brake force booster with the aid of electromagnetically controllable multi-way valves. The hydraulic pressure in the master brake cylinder, downstream of the brake force booster, is reduced by that counter-force. Through equally electromagnetically controllable multi-way valves provided in the hydraulic conduits leading to the individual wheel brakes, the previously attained pressure in the wheel brakes, is maintained or reduced to the pressure level in the master brake cylinder decreased by the said counter-acting force. The pressure in the individual wheel brakes is cyclically successively adjusted to the valve computed by the electronic controller according to the so-called multiplex process.
In the state-of-the-art brake systems it is difficult to vary the brake pressure in the individual wheel brakes sufficiently fast and to apply the auxiliary energy required when vacuum serves as an energy source for generating the brake force boosting and the restoring force acting in a direction opposite the pedal force. Moreover, the size of the servo assemblies required is restricted for cost-saving reasons and by space limitations.
The problem underlying the invention, therefore, resides in overcoming the above mentioned disadvantages while providing a brake system operating with the vacuum usually available in automotive vehicles operating with Otto engines, requiring relatively little space when it is installed and permitting comparatively low-cost manufacture. These requirements should be complied with without sacrificing the efficiency of the brakes or the anti-locking control.